Hair curvature: a natural dialectic and review

Abstract

Although hair forms (straight, curly, wavy, etc.) are present in apparently infinite variations, each fibre can be reduced to a finite sequence of tandem segments of just three types: straight, bent/curly, or twisted. Hair forms can thus be regarded as resulting from genetic pathways that induce, reverse or modulate these basic curvature modes. However, physical interconversions between twists and curls demonstrate that strict one-to-one correspondences between them and their genetic causes do not exist. Current hair-curvature theories do not distinguish between bending and twisting mechanisms. We here introduce a multiple papillary centres (MPC) model which is particularly suitable to explain twisting. The model combines previously known features of hair cross-sectional morphology with partially/completely separated dermal papillae within single follicles, and requires such papillae to induce differential growth rates of hair cortical material in their immediate neighbourhoods. The MPC model can further help to explain other, poorly understood, aspects of hair growth and morphology. Separate bending and twisting mechanisms would be preferentially affected at the major or minor ellipsoidal sides of fibres, respectively, and together they exhaust the possibilities for influencing hair-form phenotypes. As such they suggest dialectic for hair-curvature development. We define a natural-dialectic (ND) which could take advantage of speculative aspects of dialectic, but would verify its input data and results by experimental methods. We use this as a top-down approach to first define routes by which hair bending or twisting may be brought about and then review evidence in support of such routes. In particular we consider the wingless (Wnt) and mammalian target of rapamycin (mTOR) pathways as paradigm pathways for molecular hair bending and twisting mechanisms, respectively. In addition to the Wnt canonical pathway, the Wnt/Ca(2+) and planar cell polarity (PCP) pathways, and others, can explain many alternatives and specific variations of hair bending phenotypes. Mechanisms for hair papilla budding or its division by bisection or fission can explain MPC formation. Epithelial-to-mesenchymal (EMT) and mesenchymal-to-epithelial (MET) transitions, acting in collaboration with epithelial-mesenchymal communications are also considered as mechanisms affecting hair growth and its bending and twisting. These may be treated as sub-mechanisms of an overall development from neural-crest stem cell (NCSC) lineages to differentiated hair follicle (HF) cell types, thus providing a unified framework for hair growth and development.

Why is hair curly?—Deductions from the structure and the biomechanics of the mature hair shaft

“…With cell fractions being rather similar across ethnic hair types, curliness will thus primarily depend on the quality of cell segregation. In our view, this mechanism may also account for the formation of kinky hair or other highly irregular hair shapes . We suggest that over just a short fibre length enhanced cell segregation occurs.…”

“…In the cortical cells, specific keratins are synthesized which aggregate to form partly α‐helical intermediate filaments (IF) embedded in an amorphous matrix of keratin associated proteins . In the higher regions cortical cells, further differentiate into two major groups ( ortho ‐ and para ‐type cells) .…”

“…The processes in the follicle are complex, involving a large number of different factors . However, all of the processes which lead to cell differentiation, specific shape formation, and segregation in the cross section occur before the hair reaches the transition zone.…”

Human Hair and the Impact of Cosmetic Procedures: A Review on Cleansing and Shape-Modulating Cosmetics

“…Epithelial cells direct the underlying dermal cells to proliferate and form a dermal condensate, which in turn signals the epithelial cells to proliferate and grow downwards into the dermis. During cytodifferentiation, the dermal condensate is enveloped with follicular epithelial cells creating a distinct dermal papilla, which instructs the ectoderm to shape the entire hair follicle through the action of morphogens and growth factors [23,24]. …”

“…Hair can be classified generically into straight, waved, curly or kinky hair, regarding hair ethnicity into Asian, Caucasian and African hair. However, further studies analyzed hair among various world populations to classify the hair geometry based on more rigorous criteria [24,61]. De La Mettrie et al [61,62] divided the human hair types into eight classes, based on the parameter hair curve diameter, curl index and number of waves ( Figure 6).…”